Even though 2013 has featured a lower than average count of 90 degree days or higher, we aren't finished counting just yet. Global warming doesn't just affect high temperatures, it affects the length of the warm season - and there is evidence that the warm season is expanding. In a 2011 study, scientists used vegetation to track the start and end of the growing season. They found that even with great yearly variability, the growing season is lasting an average of 6.5 days longer across the northern hemisphere. This is based on 26 years of data from 1982-2008.